1. Field of the Invention
The present invention relates to an optical spectrum measuring apparatus for measuring the optical spectrum characteristics of a light source.
This application is based on patent application No. Hei 08-290853 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
A conventional optical spectrum measuring apparatus as shown in FIG. 3 will be explained below. FIG. 3 shows a light source 51, and a spectroscope 70 which is formed as a "Czerny-Turner-Type" dispersion spectroscope. This spectroscope is comprised of incident light slit 52, concave surface mirrors 53 and 55, diffraction grating 54, and outgoing light slit 56.
The apparatus also contains optical detector 57, amplification circuit 58, AD converter 59, motor 60, drive circuit 61, CPU (Central Processing Unit) 62, display section 63 and a slit control unit 65.
In this example, a beam from the light source 51 is incident on the incident light slit 52. The light is converted into a parallel beam by the concave surface mirror 53 and is incident on the diffraction grating 54.
A plurality of grooves is formed on the surface of the diffraction grating 54. The diffraction grating 54 can be rotated through an arbitrary angle around an axis parallel to these grooves by means of the motor 60. The drive circuit 61 changes the angle of the diffraction grating 54 by controlling the motor 60 according to the instructions from the CPU 62.
From the above-mentioned parallel beam, the diffraction grating 54 reflects only diffracted light of a particular wavelength component determined by the angle of diffraction grating 54, in the direction of the concave surface mirror 55.
The concave surface mirror 55 images the diffracted light onto the outgoing light slit 56.
Only the wavelength components falling within the width of the outgoing light slit 56 are able to pass through the outgoing light slit 56. At this time, slit control unit 65 sets the width of the outgoing light slit 56 according to the direction of CPU 62.
The optical detector 57 receives the light passing through the outgoing light slit 56, and converts the light into an electrical signal proportional to the optical intensity. The amplification circuit 58 amplifies the output of the optical detector 57 to a voltage appropriate to the input to the AD converter 59. The AD converter 59 converts the output of the amplification circuit 58 into a digital signal.
Following, the measuring steps, CPU 62 gives a direction to the slit control unit 65 and sets the width of the outgoing light slit 56. Next, CPU 62 gives a direction to the drive circuit 61, and sets the wavelength of the wave passing through the outgoing light slit 56 by rotating the angle of the diffraction grating 54. At this time, the strength of the light is taken from the output of the AD converter 59.
The CPU 62 sweeps the wavelength passing through the outgoing light slit 56 from a measurement initiation wavelength to a measurement termination wavelength, and displays the repetitively obtained wavelength and intensity characteristics to the display section 63 as an optical spectrum.
The band width of passing wavelength (or wavelength resolution) RB by the above "Czerny-Turner-Type" dispersion spectroscope is represented approximately as follows, under the condition that focal length of concave surface mirror 53 and of concave surface mirror 55 are the same and that the width of outgoing light slit 56 is greater than that of incident light slit 52. EQU RB=2d/(m.multidot.f).multidot.S.multidot.cos .beta. (1)
d is distance of ditch of the diffraction grating 54, m is diffraction degree of the diffraction gating 54, f is focal length of concave surface mirror 53 and 55, and .beta. is an angle of the diffraction light of the diffraction gating 54 and the normal of the diffraction gating 54.
By rotating the diffraction gating 54 in order to vary the wavelength of the wave, .beta. changes. This means the band width of the wavelength changes depending on the wavelength of the wave on the basis of the formula (1).
In order to use the spectroscope 70 for a wide range of the wavelength, diffraction degree of the diffraction gating 54 may be changed. For an example of using the diffraction gating 54 with 900 ditches per one mm, secondary light is used in the range of wavelength of 350 nm.about.600 nm and primary light is used in the range of wavelength of 600 nm.about.1750 nm. In this case, the width of outgoing light slit 56 is selected according to the degree of changed diffraction gating 54 to be able to obtain the intended band width of the wavelength for the respective diffraction degree.
The width of the outgoing light slit 56 is determined to obtain the designed resolution at the center of the wavelength measured for each diffraction degree. This results in the band width of the wavelength having a difference between the case of the secondary light and of the primary light.
The above example is designed so that the intended band width of the wavelength is obtained at the wavelength of 550 nm when the secondary light is used, and at 1350 nm when the primary light is used.
FIG. 4 shows the wavelength characteristic of the bandwidth of the wavelength. In the above example, the bandwidth of the wavelength at 600 nm which is a switching point of the degree is 8.5 nm for the secondary light, and is 13 nm for the primary light.
The spectrum of the light to be measured is generally broader than the bandwidth of the wavelength. In other words, in the example with a characteristic of the bandwidth of the wavelength shown in FIG. 4, there is a problem in that the measured optical spectrum has undesirable characteristics of higher values in the range of short wavelengths. There is also a problem in that the optical spectrum is discontinuous at the switching point of the degree.
FIG. 5 shows the result of measurement of the LED optical spectrum using a optical spectrum measuring apparatus with a wavelength of 660 nm as a light source 51. As shown in this FIG. 5, the optical spectrum is discontinuous at the point A.